Zn4Sb3(C7) powders as a potential anode material for lithium-ion batteries

The electrochemical properties of β-Zn₄Sb₃ and Zn₄Sb₃C₇ as new lithium-ion anode materials were investigated. The reversible capacities of the pure Zn₄Sb₃ alloy electrode and 100 h milled Zn₄Sb₃ in the first cycle reached 503 and 566 mA h/g, respectively, but the cycle stability of Zn₄Sb₃ whether milled or not were obviously bad. It was demonstrated that cycle stability of Zn₄Sb₃ could be largely improved by milling after mixing with graphite. It was shown that Zn₄Sb₃C₇ composite has a lithium-ion extraction capacity of 581 mA h/g at the first cycle and 402 mA h/g at 10th cycle.     

Series-connected tandem dye-sensitized solar cell for improving efficiency to more than 10%

As one of the methods for improving the efficiency of a dye-sensitized solar cell (DSC), we investigated series-connected tandem DSCs. In this system, the top cell is made up of a transparent cell and the bottom cell utilizes only the light passing through the top cell. We investigated several combinations of dyes in tandem-type DSCs. The best efficiency obtained in our study is 10.4% (J_sc=10.8 mA/cm², V_oc=1.45 V, and FF=0.67) for a series-connected tandem DSC consisting of an N719 top cell and a black-dye bottom cell.     

Interface properties determined the performance of thermally grown GaN/Si heterojunction solar cells

We report the fabrication of heterojunction solar cells via the thermal chemical vapor deposition (CVD) of gallium nitride (GaN) nanostructures on clean Si substrates. GaN epitaxial layers were synthesized via the direct reaction of Ga vapor and NH₃ solution at 1050 °C. The structural and optical characteristics of the as-grown GaN layers were investigated. The effects of Si orientation (100 vs 111) and doping type (n- vs p-) on the structural and optical properties of the deposited GaN nanostructures and solar cell performance were explored. The fabricated GaN nanostructures exhibited p-type behavior at the GaN/Si interface as revealed from the Hall-effect measurements. The J–V characteristics showed rectifying behavior for the GaN/n-Si junction and Ohmic behavior for the GaN/p-Si junction. Upon illumination (30 mW/cm²), the as-deposited heterojunction solar cell devices showed conversion efficiencies of 6.18% and 3.69% for GaN/n-Si (1 1 1) and GaN/n-Si (1 0 0) heterojunctions, respectively. The growth of GaN on Si substrates in the presence of NH₃ solution has strong effect on the morphological, optical and electrical properties and consequently on the efficiency of the solar cell devices made of such substrates.     

The characteristics of anisotype CdS/CdTe heterojunction

Polycrystalline CdTe and CdS films were prepared by thermal evaporation technique with thicknesses 1.0 μm and 0.1 μm, respectively. The prepared films were deposited at substrate temperature 423 K, then annealed under vacuum at various annealing temperatures. Anisotype CdS/CdTe heterojunction has been prepared. The structure of the films was examined by X-ray diffraction. Hall measurements confirmed the conductivity types for CdTe and CdS to be p- and n-, respectively. Electrical characteristics of the junction (C–V and I–V measurements) showed that the junction was abrupt. Heat treatment (T_a) of the junction caused a decrease in the capacitance with increasing the reverse bias voltage. Also, both zero bias capacitance and built in voltage are decreased with increasing T_a. Carrier concentration around the junction was increased with increasing T_a. Transport mechanism of forward current coincides to tunneling-recombination mechanism; this was confirmed by I–V measurement.     

Tin and tin-based intermetallics as new anode materials for lithium-ion cells

The galvanostatic cycling behaviour of Sn/SnSb composite electrodes has been studied in 1 mol l⁻¹ LiClO₄/propylene carbonate (PC), 1 mol l⁻¹ LiPF₆/ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1), and 1 mol l⁻¹ LiClO₄/PC saturated with trans-decalin (t-Dec). Capacities between 500 and 600 mA h g⁻¹ (with respect to the mass of active material) were obtained. Reasons for the irreversible capacities are given and film formation on lithium storage metals and alloys is discussed. The observed coulombic efficiencies were slightly higher for the EC-containing electrolyte than for the PC-based one. Alternatively, improved efficiencies and stand-time behaviour were obtained when the PC electrolyte was saturated with t-Dec, which acts as a surfactant.     

Transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field design

A numerical investigation of the transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field as a function of wall temperature, fuel ratio and Reynolds number are presented. The fuel Reynolds number and H₂O/CH₃OH molar ratio (S/C) that influence the transport phenomena and methanol conversion are explored in detail. In addition, the effects of various wall temperatures on the plates that heat the channel are also investigated. The predictions show that conduction through the wall plays a significant effect on the temperature distribution and must be considered in the modeling. The predictions also indicate that a higher wall temperature enhances the chemical reaction rate which, in turn, significantly increases the methanol conversion. The methanol conversion is also improved by decreasing the Reynolds number or increasing the S/C molar ratio. When the serpentine flow field of the channel is heated either through top plate (Y = 1) or the bottom plate (Y = 0), we observe a higher degree of methanol conversion for the case with top plate heating. This is due to the stronger chemical reaction for the case with top plate heating.     

Time evolution of double-diffusive convection in a vertical cylinder with radial temperature and axial solutal gradients

The characteristics of transient double-diffusive convection in a vertical cylinder are numerically simulated using a finite element method. Initially the fluid in the cavity is at uniform temperature and solute concentration, then constant temperature and solute concentration, which are lower than their initial values, are imposed along the sidewall and bottom wall, respectively. The time evolution of the double-diffusive convection is investigated for specific parameters, which are the Prandtl number, Pr = 7, the Lewis number, Le = 5, the thermal Grashof number, Gr_T = 10⁷, and the aspect ratio, A = 2, of the enclosure. The objective of the work is to identify the effect of the buoyancy ratio (the ratio of solutal Grashof to thermal Grashof numbers: N = Gr_S/Gr_T) on the evolution of the flow field, temperature and solute field in the cavity. It is found that initially the fluid near the bottom wall is squeezed by the cold flow from the sidewall, a crest of the solute field forms and then pushed to the symmetry line. In the case of N > 0, a domain with higher temperature and weak flow (dead region) forms on the bottom wall near the symmetry line, and the area of dead region increases when N varies from 0.5 to 1.5. More crests of the solute field are formed and the flow near the bottom wall fluctuates continuously for N < 0. The frequency of the fluctuation increases when N varies from −0.5 to −1.5. Corresponding to the variety of the thermal and solutal boundary layers, the average rates of heat transfer (Nu) at the sidewall remain almost unchanged while the average rates of mass transfer (Sh) at the bottom wall change much in the cases of N = 1, 0, −1.     

Natural convection effects on heat and mass transfer in a curvilinear triangular cavity

Finite element method is used in this study to analyze the effects of buoyancy ratio and Lewis number on heat and mass transfer in a triangular cavity with zig-zag shaped bottom wall. Buoyancy ratio is defined as the ratio of Grashof number of solutal and thermal. Inclined walls of the cavity have lower temperature and concentration according to zig-zag shaped bottom wall. Enclosed space consists mostly of an absorber plate and two inclined glass covers that form a cavity. Both high temperature and high concentrations are applied to bottom corrugated wall. Computations were done for different values of buoyancy ratio (?10 ? Br ? 10), Lewis number (0.1 ? Le ? 20) and thermal Rayleigh number (10⁴ ? Ra_T ? 10⁶). Streamlines, isotherms, iso-concentration, average Nusselt and Sherwood numbers are obtained. It is found that average Nusselt and Sherwood numbers increase by 89.18% and 101.91% respectively as Br increases from ?10 to 20 at Ra_T = 10⁶. Also, average Nusselt decreases by 16.22% and Sherwood numbers increases by 144.84% as Le increases from 0.1 to 20 at this Rayleigh number.     

Mixed convective transport in a lid-driven cavity containing a nanofluid and a rotating circular cylinder at the center

The mixed convective transport of Cu-H₂O nanofluid in a differentially heated and lid-driven square enclosure in the presence of a rotating circular cylinder is investigated numerically. The top wall of the enclosure is sliding from left to right at a uniform speed while all other walls are stationary. A thermally insulated circular cylinder is placed centrally within the enclosure. The cylinder can rotate about its centroidal axis. The top and bottom walls are kept isothermal at different temperatures while the side walls are assumed adiabatic. Simulations are performed for, Richardson number 1 ≤ Ri ≤ 10, dimensionless rotational speed 0 ≤ Ω ≤ 5 and nanoparticle concentration 0 ≤ ϕ ≤ 0.20 keeping the Grashof number fixed as Gr = 10⁴. The flow and thermal fields are analyzed through streamline and isotherm plots for various Ω and Ri. Furthermore, the drag coefficient of the moving lid and Nusselt number of the hot wall are also computed to understand the effects of Ω and Ri on them. It is observed that the heat transfer greatly depends on the rotational speed of the cylinder, mixed convective strength and the nanoparticle concentration.     

Evaluation of discharge and cycling properties of skutterudite-type Co1−2yFeyNiySb3 compounds in lithium cells

The preparation, structural characterization and study of the electrochemical behavior in lithium cells of Co_1−2yFe_yNi_ySb₃ (0.125<y≤0.5) compounds is described. The refinement of X-ray diffraction (XRD) patterns evidenced the skutterudite-type structure for all compositions, with an increasing partial filling of 2a sites. The first discharge of lithium anode cells shows two plateaus. The plateau at higher potential, which occurs at ca. 0.8 V and extends from ca. 150 to 250 Ah/kg depending on “y”, is mainly assigned to side reactions of lithium with the electrolyte, as confirmed by ⁷Li NMR. The second plateau occurs at 0.5–0.6 V and is assigned to Li–Sb alloys formation. The charge of the cell shows that only the second step is reversible. Further discharge/charge cycles present a plateau at 0.8 V in the discharge and at 1.05 V in the charge, which agree well with the Li–Sb alloying/de-alloying process. Extended cycling results on a loss of capacity, to stabilize over 100 Ah/kg after 20 cycles.     

Electrohydrodynamic enhancement of in-tube convective condensation heat transfer

An experimental investigation of electrohydrodynamic (EHD) augmentation of heat transfer for in-tube condensation of flowing refrigerant HFC-134a has been performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. The effects of varying the mass flux (55 kg/m² s ⩽ G ⩽ 263 kg/m² s), inlet quality (0.2 ⩽ x_in ⩽ 0.83) and the level of applied voltage (0 kV ⩽ V ⩽ 8 kV) are examined. The heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor 1.5 at the same conditions of the maximum heat transfer enhancement. The improved heat transfer performance and pressure drop penalty are due to flow regime transition from stratified flow to annular flow as has been deduced from the surface temperature profiles along the top and bottom surfaces of the tube.     

Sintered porous heat sink for cooling of high-powered microprocessors for server applications

This paper experimentally investigates the sintered porous heat sink for the cooling of the high-powered compact microprocessors for server applications. Heat sink cold plate consisted of rectangular channel with sintered porous copper insert of 40% porosity and 1.44 × 10^?11 m² permeability. Forced convection heat transfer and pressure drop through the porous structure were studied at Re ? 408 with water as the coolant medium. In the study, heat fluxes of up to 2.9 MW/m² were successfully removed at the source with the coolant pressure drop of 34 kPa across the porous sample while maintaining the heater junction temperature below the permissible limit of 100 ± 5 °C for chipsets. The minimum value of 0.48 °C/W for cold plate thermal resistance (R_cp) was achieved at maximum flow rate of 4.2 cm³/s in the experiment. For the designed heat sink, different components of the cold plate thermal resistance (R_cp) from the thermal footprint of source to the coolant were identified and it was found that contact resistance at the interface of source and cold plate makes up 44% of R_cp and proved to be the main component. Convection resistance from heated channel wall with porous insert to coolant accounts for 37% of the R_cp. With forced convection of water at Re = 408 through porous copper media, maximum values of 20 kW/m² K for heat transfer coefficient and 126 for Nusselt number were recorded. The measured effective thermal conductivity of the water saturated porous copper was as high as 32 W/m K that supported the superior heat augmentation characteristics of the copper–water based sintered porous heat sink. The present investigation helps to classify the sintered porous heat sink as a potential thermal management device for high-end microprocessors.     

Modeling heat transfer in Bi2Te3–Sb2Te3 nanostructures

Bi₂Te₃–Sb₂Te₃ nanostructures are gaining importance for use in thermoelectric applications following the finding that the Bi₂Te₃–Sb₂Te₃ superlattice exhibits a figure of merit, ZT = 2.4, which is higher than conventional thermoelectric materials. In this paper, thermal transport in the cross-plane direction for Bi₂Te₃–Sb₂Te₃ nanostructures is simulated using the Boltzmann transport equation (BTE) for phonon intensity. The phonon group velocity, specific heat, and relaxation time are calculated based on phonon dispersion model. The interfaces are modeled using a combination of diffuse mismatch model (DMM), and the elastic acoustic mismatch model (AMM). The thermal conductivity for the Bi₂Te₃–Sb₂Te₃ superlattice is compared with the experimental data, and the best match is obtained for specularity parameter, p, of 0.9. The present model is extended to solve for thermal transport in 2-D nanowire composite in which Sb₂Te₃ wires are embedded in a host material of Bi₂Te₃. Unlike in bulk composites, the results show a strong dependence of thermal conductivity, temperature, and heat flux on the wire size, wire atomic percentage, and interface specularity parameter. The thermal conductivity of the nanowire is found to be in the range of 0.034–0.74 depending on the atomic percentage and the value of p.     

Heat transfer enhancement of nanofluids in a double pipe heat exchanger with louvered strip inserts

The effect of using louvered strip inserts placed in a circular double pipe heat exchanger on the thermal and flow fields utilizing various types of nanofluids is studied numerically. The continuity, momentum and energy equations are solved by means of a finite volume method (FVM). The top and the bottom walls of the pipe are heated with a uniform heat flux boundary condition. Two different louvered strip insert arrangements (forward and backward) are used in this study with a Reynolds number range of 10,000 to 50,000. The effects of various louvered strip slant angles and pitches are also investigated. Four different types of nanoparticles, Al₂O₃, CuO, SiO₂, and ZnO with different volume fractions in the range of 1% to 4% and different nanoparticle diameters in the range of 20 nm to 50 nm, dispersed in a base fluid (water) are used. The numerical results indicate that the forward louvered strip arrangement can promote the heat transfer by approximately 367% to 411% at the highest slant angle of α = 30° and lowest pitch of S = 30 mm. The maximal skin friction coefficient of the enhanced tube is around 10 times than that of the smooth tube and the value of performance evaluation criterion (PEC) lies in the range of 1.28–1.56. It is found that SiO₂ nanofluid has the highest Nusselt number value, followed by Al₂O₃, ZnO, and CuO while pure water has the lowest Nusselt number. The results show that the Nusselt number increases with decreasing the nanoparticle diameter and it increases slightly with increasing the volume fraction of nanoparticles. The results reveal that there is a slight change in the skin friction coefficient when nanoparticle diameters of SiO₂ nanofluid are varied.     

Rayleigh–Bénard convection of Al2O3/water nanofluids in a cavity considering sedimentation,thermophoresis, and Brownian motion

This paper aims to investigate the Rayleigh–Bénard convection of Al₂O₃/water nanofluids in a cavity considering the sedimentation, thermophoresis, and Brownian motion. For this purpose, a numerical simulation is carried out using finite volume method to obtain the flow patterns and heat transfer characteristics of nanofluid flow inside the cavity. The results are presented for volume fractions up to 4%, Rayleigh numbers between 10⁴ and 8 × 10⁴ where the temperature difference between top and bottom walls is between 2 and 10 °C. To validate the numerical solution, comparisons are made between the numerical results and experimental data available in the literature. The comparisons unfold that the results of numerical work are in a good agreement with the previous experimental results.     

Characterization of a scroll compressor under extended operating conditions

Refrigeration and air-conditioning compressors are designed to work under well-defined conditions. In some applications it is interesting to observe their performances beyond these conditions, for example in the case of a high temperature two-stage heat pump or of a cooling system working at high temperature.In this study a compressor is characterized experimentally with refrigerant R134a and through 118 tests at condensing pressures varying from 8.6 up to 40.4 bar (t_sat = 33.9 °C to t_sat = 100.8 °C) and evaporating pressures varying from 1.6 up to 17.8 bar (t_sat = ?15.6 °C to t_sat = 62.4 °C). Under these conditions the compressor motor was pushed at its maximal current in several tests.This compressor’s performance is mainly characterized by its isentropic and volumetric efficiencies. It presents a maximal isentropic efficiency of 72%, corresponding to a pressure ratio of around 2.5–2.6. The volumetric efficiency decreases linearly from almost 1.0 (for a pressure ratio of 1.3) to 0.83 (for a pressure ratio of 9.7). A slight degradation of the isentropic and volumetric efficiencies is observed when the compressor supply and exhaust pressures are increased for a given pressure ratio; this could be due to an internal leakage.The compressor tests are used to identify the six parameters of a semi-empirical simulation model. After parameter identification, experimental and simulated results are in very good agreement, except for some points at high compressor power where the compressor is pushed at its maximal current.     

Negative pressure dependence of mass burning rates of H2/CO/O2/diluent flames at low flame temperatures

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H₂/CO/O₂/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO₂ up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO₂ was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N₂ and H₂O.Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO_x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO₂ + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O₂(+M) = HO₂(+M) in both pure and mixed bath gases, in rate constants for HO₂ reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H₂ or H₂/CO flames.     

Spray water cooling heat transfer at high temperatures and liquid mass fluxes

Spray water cooling is an important technology used in industry for the cooling of materials from temperatures up to 1800 K. The heat transfer coefficient in the so-called steady film boiling regime is known to be a function of the water impact density. Below a specific surface temperature T_L, the heat transfer coefficient shows a strong dependence on temperature (Leidenfrost effect). These findings are the results of complex self-organizing two-phase boiling heat transfer phenomena.The heat transfer coefficient was measured by an automated cooling test stand (instationary method) under clean (non-oxidizing) surface conditions. Compared to the common thought, an additional temperature dependency in the high temperature regime was found. The heat transfer from the material to the outflowing spray water is explained by a simple model of the two-phase flow region. From the experimental data, an analytic correlation for the dependence of the heat transfer coefficient α as an analytic function of water impact density V_S and temperature ΔT is provided.For water temperatures around 291 K, surface temperatures between 473 and 1373 K, i.e. ΔT > 180 K and water impact densities between V_S = 3 and 30 kg/(m² s) the heat transfer coefficient α was measured. The spray was produced with full cone nozzles (v_d ≈ 13–15 m/s, d_d ≈ 300–400 μm).     

Self-ignition combustion synthesis of TiFe1−xMnx hydrogen storage alloy

The paper describes the self-ignition combustion synthesis (SICS) of the hydrogen storage alloy TiFe_1?xMn_x (X = 0, 0.1, 0.2, 0.3, and 0.5) in a hydrogen atmosphere, where the hydrogenation properties of the products are mainly examined. In the experiments, the well-mixed powders of Ti, Fe, and Mn in the molar ratio of 1:1-X:X were uniformly heated up to 1473 K, and then were cooled naturally in pressurized hydrogen at 0.9 MPa. All products were successfully synthesized by utilizing the exothermic reaction, which occurred at around 1358 K. The XRD analysis showed that SICS generated TiFe_1?xMn_x in the range of X value from 0 to 0.3. All SICSed products absorbed hydrogen smoothly at 298 K at an initial pressure of 4.1 MPa. Most significantly, TiFe_0.8Mn_0.2 improved the dual plateau property. The results revealed that SICS was quite effective for producing the hydrogen storage alloy TiFe_1?xMn_x.